Scenarios to Investigate the Effect ofWetland Position in a Watershed on Nutrient Loadings

Abstract

Lake Winnipeg is Canada’s 6th largest freshwater lake and is subject to an increasing rate of eutrophication as a result of nonpoint source pollution from numerous sources including farms and municipal wastewaters. We examine scenarios designed to compare the relative effects of wetland restoration and position on modelled nutrient loadings to Lake Winnipeg from a pilot watershed in the Lake’s Basin, the La Salle River watershed. Scenarios were examined using the Soil and Water Assessment Tool (SWAT). SWAT is a well-known watershed scale hydrologic model designed to assess non-point source pollution loadings to contributing streams across a wide range of scales. Modelled results suggested that increasing wetland cover to historic levels decreased yearly nutrient loadings by 9-21% for both TN and TP. When 25% of historic wetland areas were restored at subwatershed outlets, equivalent or better nutrient reductions were attained. But placing all wetland area at the watershed outlet did not result in as substantial nutrient reductions. There was a larger range of uncertainty when wetlands were modelled across all subwatersheds than when the entire wetland area was modelled at the outlet. These results may indicate that wetland position is as important as wetland amount in terms of nutrient reductions.